Skip to main content

Full text of "Handbook Of Chemical Engineering - I"

See other formats


EVAPORATION                                          381
chemicals or mechanical cleaning.    The horizontal-type machine is not very suitable as the tubes cannot be cleaned mechanically.
Evaporators are usually built with cast-iron shells and copper tubes. In some cases evaporators have been built of copper entirely, but experience has shown that this is not necessary in order to make a high-quality extract.
Milk.—For the manufacture of condensed or evaporated milk, skimmed milk is usually concentrated to M or K of its original volume, and this is done in single-effect evaporators of the film or coil type. The capacity is from 2 to 3 gal. per square foot, with low-pressure steam, and up to 4 gal. per square foot with high-pressure steam, at a vacuum of from 26 to 28 in. So as not to cook the milk, the temperature must not exceed 60°C., and in Europe skimmed milk has been concentrated in double-effect evaporators of the rapid-circulation type with good success.
Milk-whey is concentrated to recover the milk sugar after the albuminous matter has been removed by acidification. The filtered liquor is concentrated from 23^ to 20°Be*. in double-effect evaporators of the horizontal or rapid-circulation type, with a capacity of from 2J^ to 3 gal. per square foot, with exhaust steam and from 26 to 28 in. vacuum. This solution is further concentrated to about 40°Be*. in a single effect of the coil type, having a capacity of about % £&!• Per square foot. In all cases special precautions must be taken to avoid losses by foaming, and the rapid-circulation type has been very successful on this account. Milk-whey will cause a scaling of the tubes and frequent cleaning is necessary. Evaporators for condensed or evaporated milk should be built of copper, and all parts must be accessible for frequent cleaning and scouring. Milk-whey evaporators are built with cast-iron shells and copper tubes.
Glue.—Hide-glue is usually concentrated from 2 to 15 per cent solids, and bone glue from 3 to 20 per cent in a horizontal-tube or rapid-circulation type double or triple effect evaporator. The capacity is from 2 to 2>^ gal. per square foot with a steam pressure of 5 Ib. and a vacuum of 27 in. Evaporators are built of cast iron with copper or brass tubes.
Gelatin.—Methods of manufacture vary a great deal, but usually the concentration is from 4 to 10 per cent, and the capacity of the evaporators may be figured as 3 gal. per square foot with a steam pressure of 0 Ib. and a vacuum of 27 to 28 in. Only single effects or double effects may be used, as higher temperatures will spoil the product. Ordinary gelatin will require evaporators with cast-iron shells and copper tubes, the same as for glue. High-grade gelatin should be made in machines built of copper, monel metal or nickel.
Beef Extract.—Depending on the nature of the extract to be made, the points of concentration will vary and may be assumed from 2 to 10 per cent, and also from 4 to about 60 per cent. The capacity will vary from 2 to 3 gal. per square foot with 5 Ib. steam pressure and from 26 to 27 in. of vacuum. The horizontal-type machine is used for this purpose and is built with cast-iron shell and copper tubes.
Tanning Extract.—The strength of the weak liquors varies with the raw material. The leaching of chestnut chips will give extracts containing from 3 to 4 per cent solids, while Quebracho and Campeche chips will give liquors containing from 7 to 10 per cent of solids. They are usually concentrated to about 50 per cent in a multiple-effect evaporator, with a capacity of from 3 to 4 gal. per square foot and a steam pressure of 5 Ib. and a vacuum of 27 to 28 in. The 50 per cent extract is sometimes concentrated further in special single-effect